| Floater | |
|---|---|
| Classification and external resources | |
Simulated image of floaters against a blue sky |
|
| ICD-10 | H43.9 |
| ICD-9 | 379.24 |
| DiseasesDB | 31270 |
| MedlinePlus | 002085 |
Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent.[1][2] At young age the vitreous is perfectly transparent, but during life imperfections gradually develop. The common type of floater, which is present in most people's eyes, is due to degenerative changes of the vitreous humour. The perception of floaters is known as myodesopsia,[3] or less commonly as myiodeopsia, myiodesopsia, or myodeopsia.[1] They are also called Muscae volitantes (from the Latin, meaning 'flying flies'), or mouches volantes (from the French). Floaters are visible because of the shadows they cast on the retina[4] or their refraction of the light that passes through them, and can appear alone or together with several others in one's field of vision. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the observer's eyes.[2] Since these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena.
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Floaters are suspended in the vitreous humour, the thick fluid or gel that fills the eye.[5] Thus, they generally follow the rapid motions of the eye, while drifting slowly within the fluid. When they are first noticed, the natural reaction is to attempt to look directly at them. However, attempting to shift one's gaze toward them can be difficult since floaters follow the motion of the eye, remaining to the side of the direction of gaze. Floaters are, in fact, visible only because they do not remain perfectly fixed within the eye. Although the blood vessels of the eye also obstruct light, they are invisible under normal circumstances because they are fixed in location relative to the retina, and the brain "tunes out" stabilized images due to neural adaptation. This stabilization is often interrupted by floaters, especially when they tend to remain visible.[2]
Floaters are particularly noticeable when looking at a blank surface or an open monochromatic space, such as blue sky. Despite the name "floaters", many of these specks have a tendency to sink toward the bottom of the eyeball, in whichever way the eyeball is oriented; the supine position (looking up or lying back) tends to concentrate them near the fovea, which is the center of gaze, while the textureless and evenly lit sky forms an ideal background against which to view them.[5] The brightness of the daytime sky also causes the eyes' pupils to contract, reducing the aperture, which makes floaters less blurry and easier to see.
Floaters are essentially changeless, and the most prominent continue to be seen in the field of vision for a lifetime.[6] They are not uncommon, and do not cause serious problems for most people; they represent one of the most common presentations to hospital eye services. A survey of optometrists in 2002 suggested that an average of 14 patients per month per optometrist presented with symptoms of floaters in the UK.[7] However, floaters are more than a nuisance and a distraction to those with severe cases, especially if the spots seem to constantly drift through the field of vision. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour. Floaters can even be seen when the eyes are closed on especially bright days, when sufficient light penetrates the eyelids to cast the shadows. It is not, however, only elderly people who are troubled by floaters; they can also become a problem to younger people, especially if they are myopic. They are also common after cataract operations or after trauma. In some cases, floaters are congenital.[8]
Floaters are able to catch and refract light in ways that somewhat blur vision temporarily until the floater moves to a different area. Often they trick people who are troubled by floaters into thinking they see something out of the corner of their eye that really is not there. Most people come to terms with the problem, after a time, and learn to ignore their floaters. For people with severe floaters it is nearly impossible to completely ignore the large masses that constantly stay within almost direct view.
Floaters have been reported in patients as young as 6. Floaters in teenage patients and young adults are usually harder to treat. For people in this age group, the floater that is seen usually looks like a kind of crystal (translucent) worm/web/cell. These particular floaters aren't really floaters in a technical sense as they aren't found in the vitreous humour, instead they are found right on top of the retina in the Premacular Bursa. Very little is known about this region, and it only becomes distinct after the vitreous detaches from the retina at later stages of life. Due to their microscopic size they cannot be seen by professional doctors. They only appear as big as they do because of their proximity to the retina. These types of floaters are still described occasionally in the third decade and very rarely occur in the 40 or older population.[9][10]
There are various causes for the appearance of floaters, of which the most common are described here. Simply stated, any damage to the eye that causes material to enter the vitreous humour can result in floaters.
There are websites that advise that causes of floater include, but may not be limited to- rubbing the eye, sleeping on the back or front, smoking, using eye drops or medication or illegal drugs. There is NO evidence to suggest that any of the above causes floaters. Floaters can be a sign of retinal detachment or a retinal tear but in most (98% according to the National Health Service) cases it is simply age related or due to natural change in the vitreous humour.
The most common cause of floaters is shrinkage of the vitreous humour: this gel-like substance consists of 99% water and 1% solid elements. The solid portion consists of a network of collagen and hyaluronic acid, with the latter retaining water molecules. Depolymerization of this network makes the hyaluronic acid release its trapped water, thereby liquefying the gel. The collagen breaks down into fibrils, which ultimately are the floaters that plague the patient. Floaters caused in this way tend to be few in number and of a linear form.
In time, the liquefied vitreous body loses support and its framework contracts. This leads to posterior vitreous detachment, in which the vitreous body is released from the sensory retina. During this detachment, the shrinking vitreous can stimulate the retina mechanically, causing the patient to see random flashes across the visual field, sometimes referred to as "flashers", a symptom more formally referred to as photopsia. The ultimate release of the vitreous around the optic nerve head sometimes makes a large floater appear, usually in the shape of a ring ("Weiss ring").[11] As a complication, part of the retina might be torn off by the departing vitreous body, in a process known as retinal detachment. This will often leak blood into the vitreous, which is seen by the patient as a sudden appearance of numerous small dots, moving across the whole field of vision. Retinal detachment requires immediate medical attention, as it can easily cause blindness. Consequently, both the appearance of flashes and the sudden onset of numerous small floaters should be rapidly investigated by an eye care provider.[12]
The hyaloid artery, an artery running through the vitreous humour during the fetal stage of development, regresses in the third trimester of pregnancy. Its disintegration can sometimes leave cell matter.[13]
According to the UK 'Yellow Card Scheme' that allows people to report adverse effects of prescription drugs other than those stated, there is one possible case of Ocular Zovirax causing floaters. This is the first report of such a reaction though the drug is frequently used to treat ocular herpes.
Patients with retinal tears may experience floaters if red blood cells are released from leaky blood vessels, and those with a posterior uveitis or vitritis, as in toxoplasmosis, may experience multiple floaters and decreased vision due to the accumulation of white blood cells in the vitreous humour.[14]
Other causes for floaters include cystoid macular edema and asteroid hyalosis. The latter is an anomaly of the vitreous humour, whereby calcium clumps attach themselves to the collagen network. The bodies that are formed in this way move slightly with eye movement, but then return to their fixed position.
Sometimes the appearance of floaters has to be attributed to dark specks in the tear film of the eye. Technically, these are not floaters, but they do look the same from the viewpoint of the patient. People with blepharitis or a dysfunctional meibomian gland are especially prone to this cause, but ocular allergies can also cause the problem. To differentiate between material in the vitreous humour of the eye and debris in the tear film, one can look at the effect of blinking: debris in the tear film will move quickly with a blink, while floaters are largely unresponsive to it. Tear film debris is diagnosed by eliminating the possibility of true floaters and macular degeneration.
Floaters are often readily observed by an ophthalmologist or an optometrist with the use of an ophthalmoscope or slit lamp. However, if the floater is near the retina, it may not be visible to the observer even if it appears large to the sufferer.
Increasing background illumination or using a pinhole to effectively decrease pupil diameter may allow a person to obtain a better view of his or her own floaters. The head may be tilted in such a way that one of the floaters drifts towards the central axis of the eye. In the sharpened image the fibrous elements are more conspicuous. [15]
The presence of retinal tears with new onset of floaters was surprisingly high (14%; 95% confidence interval, 12%-16%) as reported in a metaanalysis published as part of the Rational Clinical Examination Series in the Journal of the American Medical Association.[16] Patients with new onset flashes and/or floaters, especially when associated with visual loss or restriction in the visual field, should seek more urgent ophthalmologic evaluation.
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